Method and facility for producing oxygen through air distillation

ABSTRACT

In a method for producing oxygen through the distillation of air supplied by air at atmospheric pressure so as to produce a first and second compressed air flow, and through a first purification unit ( 5 ) and a second purification unit ( 7 ), the first and second compressed air flows at different pressures: the first compressed air flow is sent from a first outlet of the compression means to the first purifying unit at the first pressure; the first purified air flow is sent to a column of the column system ( 15 ); the second purified air flow is sent, in an at least partially condensed form, from the second purifying unit to the column system; an oxygen-rich liquid is bled off the column system; said oxygen-rich liquid is vaporized through heat exchange with at least the second purified air flow.

The present invention relates to a method and to a facility for producing oxygen by distilling air. The invention applies for example to the production of very large quantities of oxygen in which the oxygen pressure required is in a range comprised, for example, between 5 and 20 bar. The oxygen is produced in one or more large-sized air distillation units in which it is advantageous for the liquid oxygen produced in the distillation unit(s) to be brought to these pressures using pumps and for the liquid oxygen to be vaporized by exchange of heat with a calorigenic fluid compressed to a pressure sufficient to allow the oxygen to vaporize, this calorigenic fluid typically being pressure boosted air. The always tricky use of oxygen compressors is thus avoided.

It is common practice in such air separation units (ASUs) for air to be compressed at atmospheric pressure in one or more main air compressor(s) installed in parallel. The air thus compressed is cooled by refrigeration means, typically in a range comprised for example between 5 and 40° C. The air thus cooled is processed in one or more purification unit(s) in which impurities such as water, CO₂ and hydrocarbons are, for the most part, eliminated.

Some of this air thus purified is sent to a pressure booster where it undergoes an additional compression step, typically to beyond 10 bar, and for example constitutes a calorigenic fluid used to vaporize the product or products such as oxygen.

The production of large quantities of oxygen by ASUs entails purifying large quantities of air in the purification units and in order to do that minimizing the size of these purification units that are able to process a given volume of air.

The use of purification units of the concentric bed type makes it possible to reduce the size of these units, something that can also be obtained by increasing the pressure of the purified air in these units, or by lowering the temperature thereof.

U.S. Pat. No. 5,337,570 describes a method in which two air flows are purified at different pressures, but one of these flows then has its pressure boosted to a higher pressure so as to be able to vaporize a pressurized liquid oxygen flow.

The present invention seeks to alleviate the defects of the prior art and may make it possible to reduce the cost of investments by avoiding the addition of any air pressure booster after the purification unit(s) and instead to have equivalent compression prior to the step of purifying the air in the purification unit(s).

The purification units will process two air flows at two different pressures, the first air flow at a first pressure of between 5 and 9 bar or potentially of between 2 and 4 bar and the second air flow at a second pressure of between 11 and 50 bar or potentially of between 4.5 and 8 bar.

One subject of the invention is a method for producing oxygen by distilling air in an apparatus comprising at least one system of columns, at least one exchange line, at least one compression means driven by an electric motor and/or by a steam turbine and supplied with air at atmospheric pressure to produce one first and one second pressurized air flow, one first purification unit, one second purification unit, the first and second pressurized air flows leaving the compression means at a first and a second pressure, the second pressure being higher than the first by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, even by at least 25 bar, and the second pressure being the highest pressure of any air flow intended to be fed into the system of columns; in which method the first pressurized air flow is sent from a first outlet of the compression means to the first purification unit substantially at the first pressure in order to produce a first air flow that is purified in terms of water and in terms of carbon dioxide, the second pressurized air flow is sent from a second outlet of the compression means to the second purification unit substantially at the second pressure in order to produce a second air flow that is purified in terms of water and in terms of carbon dioxide, the purified first flow is cooled in the exchange line, possibly at the first pressure, the purified second flow is cooled in the exchange line, possibly at the second pressure, the purified first air flow is sent from the first purification unit to one column of the system of columns, the purified second air flow from the second purification unit is sent to one column of the system of columns at least in partially condensed form, an oxygen-rich liquid is tapped off from the system of columns, vaporized, possibly in the exchange line or in an auxiliary vaporizer by exchanging heat at least with the purified second flow at the second pressure and is delivered as product.

According to other subjects of the invention;

-   -   The difference in pressure between the two pressurized air flows         is at most 4 bar or possibly at least 1 bar and at most 3 bar.     -   The difference in pressure between the two pressurized air flows         is at least 5 bar and at most 30 bar, or possibly at least 15         bar and at most 25 bar.     -   At least some of the purified first air flow is sent into the         same column of the system of columns as the purified second air         flow.     -   No part of the second flow is sent into a reboiler of the system         of columns.     -   The second pressure is higher than the first by at least 5 bar.     -   The second pressure is high than the first by at least 10 bar.     -   The second pressure is higher than the first by at most 25 bar.     -   The flow at the second pressure enters one column of the system         of columns and is not used to heat a reboiler of the system of         columns.

Another subject of the invention is a facility for producing oxygen by distilling air, comprising at least one system of columns, at least one exchange line, at least one compression means driven by a steam turbine and/or by an electric motor, the compression means having a first and a second outlet, one first purification unit, one second purification unit, the compression means being designed to be supplied with air at atmospheric pressure and to produce, from the first outlet, a first pressurized air flow at a first pressure and from the second outlet a second pressurized air flow at a second pressure, the second pressurized air flow being at a pressure that is higher, by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, even by at least 25 bar, than the pressure of the first pressurized air flow, a first pipe for connecting the first outlet to the first purification unit, a second pipe for connecting the second outlet to the second purification unit, a third pipe for connecting the first purification unit to the exchange line, a fourth pipe for connecting the second purification unit to the exchange line, no means of boosting the air pressure being connected downstream of the second purification unit, a fifth pipe connecting the exchange line to one column of the system of columns, a sixth pipe for connecting the exchange line to one column of the system of columns, a pipe for tapping off an oxygen-rich liquid from the system of columns and for sending to a vaporizer (25), possibly consisting of the exchange line, means for sending at least part of the purified second flow to the vaporizer where it condenses and in which facility there is no means of compressing air between the first outlet and the first purification unit and there is no means of compressing air between the second outlet and the exchange line, or potentially the system of columns.

According to other aspects of the invention:

-   -   The compression means comprises a first compressor and a second         compressor, means for supplying the first compressor and the         second compressor with air at atmospheric pressure, the first         and second compressors possibly being driven by a common steam         turbine.     -   Just one of the first and second air compressors comprises         intermediate coolants (isothermal compression).     -   Means for sending air from the outlet of that one of the two air         compressors that does not have an intermediate coolant to a heat         exchanger, and means for sending at least one fluid from the         system of columns and/or water to the exchanger where it is         heated up.

Another subject of the invention is a method of producing oxygen by distilling air in an apparatus comprising n systems of columns, where n≧2, n exchange lines, at least one first compressor compressing atmospheric air in order to produce an air flow at a first pressure, at least one second compressor compressing atmospheric air to produce an air flow at a second pressure, the first pressure lower by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, or even by at least 25 bar, than the second pressure, and the second pressure being the highest pressure of any air pressure intended for distillation, in which method air at the first pressure is sent from at least one first compressor to at least one first purification unit, air at the second pressure is sent from at least one second compressor to at least one second purification unit, air at the first pressure is sent from the first purification unit to at least two systems of columns, air at the second pressure is sent from the second purification unit to at least two systems of columns, and oxygen is produced from at least one of the systems of columns.

Another subject of the invention is a facility for producing oxygen by distilling air in an apparatus comprising n systems of columns, where n≧2, n exchange lines, at least one first compressor compressing atmospheric air in order to produce an air flow at a first pressure, at least one second compressor compressing atmospheric air to produce an air flow at a second pressure, the first pressure lower by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, or even by at least 25 bar, than the second pressure, at least one first purification unit, at least one second purification unit, means for sending air at the first pressure taken from the first compressor(s) to the first purification unit(s), means for sending air at the second pressure taken from the second compressor(s) to the second purification unit(s), means for sending air to at least two systems of columns from the first purification unit(s) and means for sending air to the two systems of columns from the second purification unit(s), in which facility there is no compression means between the first compressor(s) and the first purification unit(s) and there is no compression means between the second compressor(s) and the exchange lines, or potentially the systems of columns.

For preference, there is no means of connecting the outlet of (one of) the first compressor(s) to the outlet of (one of) the second compressor(s) and/or there is no means of connecting the outlet of (one of) the first purification unit(s) to the outlet of (one of) the second purification unit(s).

Thus, there is an independent circuit supplied by at least two compressors producing air at the first pressure and an independent circuit supplied by at least two compressors producing air at the second pressure, each of the two circuits supplying at least two independent systems of columns.

Some embodiments will now be described in relation to the attached drawings which depict air separation facilities according to the invention.

The facility depicted in FIG. 1 is intended to supply oxygen to one or more iron smelting-reduction unit(s) (Corex®/Finex®) or to one or more oxycombustion unit(s) for example. In the first case, the pressure of the oxygen supplied is comprised in a range from 5 to 15 bar. In the second case, the pressure of the oxygen supplied is comprised in a range of 1 to 5 bar (preferably 1 to 2 bar abs).

The facility comprises one first compressor 1 and one second compressor 3, installed on the same site, means for supplying the first compressor and the second compressor with air at atmospheric pressure, the first and the second compressor being driven by electric motors and respectively bringing the air to a first pressure comprised between 2.5 and 8 bar and to a second pressure comprised between 4 and 30 bar.

The two separate compressed air flows leaving the two air compressors are cooled for example using a final coolant, before being sent into a first and a second purification unit 5 and 7, one of the air flows being substantially at the first pressure and the second substantially at the second pressure.

The purified first air flow is sent into the main exchange line 13 by means of the pipe 11 and the purified second air flows is sent into the main exchange line 13 by means of the pipe 9.

Once it has been cooled in the exchanger 13, the first air flow is introduced into the system of columns 15, the second air flow is introduced into the system of columns 15 in at least partially condensed form after having passed through an auxiliary vaporizer 25 using an oxygen-rich liquid tapped off from the system of columns 15 by means of a pipe 17 and a pump 23. The first air flow introduced into the system of columns 15 is at least partially introduced into the same column as the second air flow introduced at least partially condensed into the system of columns 15 (for example the high-pressure column of a double column comprising a high-pressure column and a low-pressure column).

FIG. 2 illustrates a first alternative form of this facility in which just one of the first and second air compressors comprises intermediate coolants (isothermal compression) namely the compressor 1, means for sending air taken from the outlet of that one of the two air compressors that does not comprise an intermediate coolant to a heat exchanger 31, and means for sending at least one fluid taken from the system of columns and/or water to the exchanger where it is heated up.

The two compressed air flows leaving the two air compressors are sent into two purification units 5 and 7, one of them substantially at the first pressure and the second substantially at the second pressure.

The purified first air flow is sent into the main exchange line 13 by means of pipes 11 and the purified second air flow is sent into the main exchange line 13 by means of the pipe 9.

Once it has been cooled in the exchanger 13, the first air flow is introduced into the system of columns 15, the second air flow is introduced into the system of columns 15 in at least partially condensed form after having passed through an auxiliary vaporizer 25 using an oxygen-rich liquid tapped off from the system of columns 15 by means of a pipe 17 and a pump 23. The first air flow introduced into the system of columns 15 is at least partially introduced into the same column as the at least partially condensed second air flow 15. The oxygen-rich liquid tapped off from the system of columns 15 by means of the pipe 17 and which was vaporized in the auxiliary vaporizer 25 against the purified second air flow, is introduced into the heat exchanger 31 and allows the cooling of the air compressed in the compressor 1 comprising no intermediate coolants.

The facility depicted in FIG. 3 represents a second variant, intended for supplying oxygen to an iron smelting-reduction unit (Corex®/Finex®). The pressure of the oxygen supplied is comprised in a range from 5 to 15 bar (preferably from 8 to 12 bar abs).

The facility comprises a first compressor 1 and a second compressor 3, means for supplying the first compressor and the second compressor with air at atmospheric pressure, the first and second compressors being driven by a common steam turbine 39 and respectively bringing the air to a first pressure of between 4 and 7 bar and to a second pressure of between 10 and 30 bar.

The two compressed air flows leaving the two air compressors are sent into two purification units 5 and 7, one of them substantially at the first pressure and the second substantially at the second pressure.

A first portion of the purified first air flow is sent into the main exchange line 13 by means of the pipes 11 and the purified second air flow is sent into the main exchange line 13 by means of the pipe 9.

The second portion of the purified first air flow is sent into the compressor 33 of a booster turbine by means of the pipe 29, before being cooled in the main exchange line 13 and then expanded in the turbine part 35 of the booster turbine. The air expanded in the turbine 35 is sent into the system of columns via the pipe 41.

The purified second air flow, once cooled in the exchange line, is introduced into the system of columns 15 by means of the pipe 43.

As in the other cases, the first air flow introduced into the system of columns 15 is introduced at least partially into the same column as the second air flow introduced at least partially condensed into the system of columns 15.

FIG. 4 illustrates a third alternative form derived from FIG. 3 in which just one of the first and second air compressors (the compressor 3) comprises intermediate coolants (isothermal compression), comprising means for sending air from the outlet of that one of the two air compressors that does not have an intermediate coolant to a heat exchanger and means for sending water to the exchanger where it heats up.

FIG. 5 describes a fourth variant of the facility described in FIG. 1, in which the two compressors are combined into one and the same machine 3, for example an axial-radial compressor.

FIG. 6 describes an additional variant in which n facilities described in FIG. 1 are interconnected. This figure, for the sake of clarity, shows the case n=2: thus it shows two facilities as described in FIG. 1, interconnected by means of the pipes 45 and 47 on the one hand, and 49 and 51 on the other. Thus, the pipe 45 connects the outlet of the compressor 1 and that of the compressor 1′, and the pipe 47 connects the outlet of the compressor 3 and that of the compressor 3′; the pipe 49 connects the outlet of the purification means 7 with that of the purification means 7′, and finally, the pipe 51 connects the outlet of the purification means 5 with that of the purification means 5′.

The first of the two interconnected facilities comprises a first and second compressor 1 and 3, the second facility comprises a first and a second compressor 1′ and 3′. The first compressors 1 and 1′ and the second compressors 3 and 3′ are supplied with air at atmospheric pressure, the first and second compressors being driven by electric motors and respectively bringing the air to a first pressure comprised between 2.5 and 8 bar and to a second pressure comprised between 4 and 30 bar.

The air flows pressurized by the compressors 1 and 1′ on the one hand, and 3 and 3′ on the other, are cooled for example using a final coolant before being sent into the first purification units 7 and 7′ on the one hand, and into the second purification units 5 and 5′ on the other, the air flows being substantially at the first pressure on the one hand in the case of the flows taken from the compressors 1 and 1′, and substantially at the second pressure on the other hand in the case of the flows taken from the compressors 3 and 3′.

The facility comprises a pipe 45 connecting the first air flows compressed by the first compressors 1 and 1′, and a pipe 47 connecting the second air flows compressed by the second compressors 3 and 3′. The facility also comprises a pipe 49 connecting the first air flows purified by the purification means 7 and 7′, and a pipe 51 connecting the second air flows purified by the purification means 5 and 5′.

The system of columns 15 in all the figures may comprise just one column, a conventional double column or a triple column with a high-pressure column, an intermediate-pressure column and a low-pressure column, amongst others. 

1-14. (canceled)
 15. A method for producing oxygen by distilling air in an apparatus comprising at least one system of columns (15), at least one exchange line (13), at least one compression means driven by an electric motor and/or by a steam turbine and supplied with air at atmospheric pressure to produce one first and one second pressurized air flow, one first purification unit (5), one second purification unit (7), the first and second pressurized air flows leaving the compression means at a first and a second pressure, the second pressure being higher than the first by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, even by at least 25 bar, and the second pressure being the highest pressure of any air flow intended to be fed into the system of columns; in which method the first pressurized air flow is sent from a first outlet of the compression means to the first purification unit (5) substantially at the first pressure in order to produce a first air flow that is purified in terms of water and in terms of carbon dioxide, the second pressurized air flow is sent from a second outlet of the compression means to the second purification unit (7) substantially at the second pressure in order to produce a second air flow that is purified in terms of water and in terms of carbon dioxide, the purified first flow is cooled in the exchange line (13), possibly at the first pressure, the purified second flow is cooled in the exchange line (13), possibly at the second pressure, the purified first air flow is sent from the first purification unit to one column of the system of columns (15), the purified second air flow from the second purification unit is sent to one column of the system of columns (15) at least in partially condensed form, an oxygen-rich liquid is tapped off from the system of columns, vaporized, possibly in the exchange line or in an auxiliary vaporizer (25) by exchanging heat at least with the purified second flow at the second pressure and is delivered as product.
 16. The method of claim 15, in which the difference in pressure between the two pressurized air flows is at most 4 bar or possibly at least 1 bar and at most 3 bar.
 17. The method of claim 15, in which the difference in pressure between the two pressurized air flows is at least 5 bar and at most 30 bar, or possibly at least 15 bar and at most 25 bar.
 18. The method of claim 15, in which at least some of the purified first air flow is sent into the same column of the system of columns as the purified second air flow.
 19. The method of claim 15, in which no part of the second flow is sent into a reboiler of the system of columns.
 20. The method of claim 15, in which the second pressure is higher than the first by at least 5 bar.
 21. The method of claim 15, in which the second pressure is higher than the first by at least 10 bar.
 22. The method of claim 15, in which the second pressure is higher than the first by at most 25 bar.
 23. A facility for producing oxygen by distilling air, comprising at least one system of columns (15), at least one exchange line (13), at least one compression means driven by a steam turbine and/or by an electric motor, the compression means having a first and a second outlet, one first purification unit (5), one second purification unit (7), the compression means being designed to be supplied with air at atmospheric pressure and to produce, from the first outlet, a first pressurized air flow at a first pressure and from the second outlet a second pressurized air flow at a second pressure, the second pressurized air flow being at a pressure that is higher, by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, even by at least 25 bar, than the pressure of the first pressurized air flow, a first pipe for connecting the first outlet to the first purification unit, a second pipe for connecting the second outlet to the second purification unit, a third pipe (11) for connecting the first purification unit to the exchange line, a fourth pipe (9) for connecting the second purification unit to the exchange line, no means of boosting the air pressure being connected downstream of the second purification unit, a fifth pipe connecting the exchange line to one column of the system of columns, a sixth pipe for connecting the exchange line to one column of the system of columns, a pipe (17) for tapping off an oxygen-rich liquid from the system of columns and for sending to a vaporizer (25), possibly consisting of the exchange line, means for sending at least part of the purified second flow to the vaporizer where it condenses and in which facility there is no means of compressing air between the first outlet and the first purification unit and there is no means of compressing air between the second outlet and the exchange line, or potentially the system of columns.
 24. The facility of claim 23, in which the compression means comprises a first compressor (1) and a second compressor (3), means for supplying the first compressor and the second compressor with air at atmospheric pressure, the first and second compressors possibly being driven by a common steam turbine (39).
 25. The facility of claim 23, in which just one of the first and second air compressors comprises intermediate coolants (isothermal compression).
 26. The facility of claim 25, comprising means for sending air from the outlet of that one of the two air compressors that does not have an intermediate coolant to a heat exchanger (31), and means for sending at least one fluid from the system of columns and/or water to the exchanger where it is heated up.
 27. A method of producing oxygen by distilling air in an apparatus comprising n systems of columns, where n≧2, n exchange lines (13, 13′), at least one first compressor (1,1′) compressing atmospheric air in order to produce an air flow at a first pressure, at least one second compressor (3,3′) compressing atmospheric air to produce an air flow at a second pressure, the first pressure being lower by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, or even by at least 25 bar, than the second pressure, and the second pressure being the highest pressure of any air pressure intended for distillation, in which method air at the first pressure is sent from at least one first compressor to at least one first purification unit (5,5′), air at the second pressure is sent from at least one second compressor to at least one second purification unit (7,7′), air at the first pressure is sent from the first purification unit to at least two systems of columns (15, 15′), air at the second pressure is sent from the second purification unit to at least two systems of columns, and oxygen is produced from at least one of the systems of columns.
 28. A facility for producing oxygen by distilling air in an apparatus comprising n systems of columns, where n≧2, n exchange lines (13, 13′), at least one first compressor (1,1′) compressing atmospheric air in order to produce an air flow at a first pressure, at least one second compressor (3,3′) compressing atmospheric air to produce an air flow at a second pressure, the first pressure being lower by at least 0.5 bar, possibly by at least 5 bar, potentially by at least 10 bar, or even by at least 25 bar, than the second pressure, at least one first purification unit (5,5′), at least one second purification unit (7,7′), means for sending air at the first pressure from the first compressor(s) to the first purification unit(s), means for sending air at the second pressure from the second compressor(s) to the second purification unit(s), means for sending air to at least two systems of columns (15, 15′) from the first purification unit(s) and means for sending air to the two systems of columns from the second purification unit(s), in which facility there is no compression means between the first compressor(s) and the first purification unit(s) and there is no compression means between the second compressor(s) and the exchange lines, or potentially the systems of columns. 